linux-kernel - Re: [PATCH 1/1] sched/cputime: Mitigate performance regression in times()/clock

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Date:	Mon, 15 Aug 2016 17:28:29 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	Stanislaw Gruszka <sgruszka@...hat.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Giovanni Gherdovich <ggherdovich@...e.cz>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Mike Galbraith <mgalbraith@...e.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Mel Gorman <mgorman@...hsingularity.net>
Subject: Re: [PATCH 1/1] sched/cputime: Mitigate performance regression in times()/clock_gettime()

2016-08-15 17:21 GMT+08:00 Stanislaw Gruszka <sgruszka@...hat.com>:
> On Mon, Aug 15, 2016 at 05:13:30PM +0800, Wanpeng Li wrote:
>> 2016-08-12 20:10 GMT+08:00 Stanislaw Gruszka <sgruszka@...hat.com>:
>> > Hi
>> >
>> > On Wed, Aug 10, 2016 at 01:26:41PM +0200, Ingo Molnar wrote:
>> >> Nice detective work! I'm wondering, where do we stand if compared with a
>> >> pre-6e998916dfe3 kernel?
>> >>
>> >> I admit this is a difficult question: 6e998916dfe3 does not revert cleanly and I
>> >> suspect v3.17 does not run easily on a recent distro. Could you attempt to revert
>> >> the bad effects of 6e998916dfe3 perhaps, just to get numbers - i.e. don't try to
>> >> make the result correct, just see what the performance gap is, roughly.
>> >>
>> >> If there's still a significant gap then it might make sense to optimize this some
>> >> more.
>> >
>> > I measured (partial) revert performance on 4.7 using mmtest instructions
>> > from Giovanni and also tested some other possible fix (draft version):
>> >
>> > diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
>> > index 75f98c5..54fdf6d 100644
>> > --- a/kernel/sched/cputime.c
>> > +++ b/kernel/sched/cputime.c
>> > @@ -294,6 +294,8 @@ void thread_group_cputime(struct task_struct *tsk, struct task_cputime *times)
>> >         unsigned int seq, nextseq;
>> >         unsigned long flags;
>> >
>> > +       (void) task_sched_runtime(tsk);
>> > +
>> >         rcu_read_lock();
>> >         /* Attempt a lockless read on the first round. */
>> >         nextseq = 0;
>> > @@ -308,7 +310,7 @@ void thread_group_cputime(struct task_struct *tsk, struct task_cputime *times)
>> >                         task_cputime(t, &utime, &stime);
>> >                         times->utime += utime;
>> >                         times->stime += stime;
>> > -                       times->sum_exec_runtime += task_sched_runtime(t);
>> > +                       times->sum_exec_runtime += t->se.sum_exec_runtime;
>>
>> If this will not have updated stats for other threads?
>
> No, until tick/sched() on CPUs running threads.

Yeah, I think this change will result in not updated stats for other
threads if they are running and before next update_curr() is called.

Regards,
Wanpeng Li